1. Field of the Invention
The present invention is in the field of devices or apparatus for detecting ferromagnetic objects in or on personnel in the vicinity of an operating magnetic resonance imaging (MRI) device.
2. Background Art
The MRI has become an invaluable tool for imaging and exploring the internal anatomy without surgery. MRI has the ability to distinguish healthy from diseased tissue, fat and muscle and adjacent structures within the body which other modalities cannot differentiate. MRI uses safe radio waves and a strong magnetic field to generate the information, which is processed by a computer, to create an image. There are now about 10,000 MRI machines currently in use in the United States.
Because of the desire for higher quality and higher resolution, the static magnetic field strength has increased steadily over the last 25 years, from a fraction of a Tesla to the current situation where 3 Tesla magnets are common. Most new magnets for MRI are superconducting and as a result very expensive to start up, causing the requirement for the magnet to be kept always on. In an emergency, these superconducting MRI magnets cannot be turned off quickly.
In an effort to allow MRI magnets with larger magnetic fields to be sited in rooms of modest dimensions, many modern MRI machines incorporate active magnetic shielding. The maximum magnetic field outside the magnet room is generally limited to 5 gauss. In order to meet this requirement, yet minimize the room size for the larger static magnetic fields of modern MRIs, these modern MRIs, use active shielding. Active shielding causes the spatial gradient (dB/dz) to increase, where B is the magnetic field strength and z is the distance from the isocenter of the MRI magnet.
The large magnetic field of the MRI magnet will attract ferromagnetic objects in the MRI magnet's field of influence. This force of attraction can cause ferromagnetic objects to approach the MRI magnet at high velocity. The force of attraction is related to the product of the spatial gradient (dB/dz) and the magnetic field strength B. Modern MRI machines, which have large magnetic fields and steep spatial gradients, can impart a strong force on relatively modest sized objects. It is known, for example, that a hairpin can reach speeds of 40 MPH when released near a typical 1.5 Tesla MRI.
The risk and consequent injuries from a projectile attracted are well known. In 2001, for example, a 6 year old boy was killed when a ferromagnetic oxygen cylinder was pulled into the magnet while being imaged. But even small objects can be dangerous. For example, an MRI patient needed to have a bobby pin surgically removed from the nasal cavity when the pin was drawn towards the center of the bore while the patient was laying head-in in the MRI. Another MRI patient forgot a fingernail clipper was in his pocket. While in the MRI magnet, he tried to remove it. The fingernail clipper slipped from his hands, and flew into his eye causing blindness. There are also life threatening risks to patients and other personnel who have implanted devices such as pacemakers, implanted defibrillators, or intracranial aneurysm clips.
Detecting small ferromagnetic objects in a passive ferromagnetic detection system is difficult since, among other factors, the disturbance to the local magnetic field is a function of the ferromagnetic mass. The detectability of a ferromagnetic object is also a function of the object's past history of exposure to magnetic fields. In the typical terrestrial environment, objects are subjected to the earth's magnetic field of approximately 0.5 Gauss.
Passive ferromagnetic detectors, meaning detectors that rely solely on the ambient magnetic field, look for distortion signatures related to the passage of magnetized objects. In addition to other factors, the degree of magnetization (M) of the object directly affects the detectability of ferromagnetic objects. The higher the level of magnetization of the object, the more easily the object is detected all other factors being equal. FIG. 1 indicates that ferromagnetic material, such as relatively pure soft iron, develops a significant degree of magnetization in the presence of modest magnetic fields such as the earth's magnetic field.
A significant degree of magnetization of a ferromagnetic object with an ambient magnetic environment provided by the 0.5 Gauss of the earth may not be produced in medium hard magnetic material such as mild steel. FIG. 2 indicates that an applied field of greater than 0.5 Gauss is required to ensure an appreciable magnetization in mild steel. If the recent magnetic history of the medium hard ferromagnetic object has only been the earth's magnetic field, the object will be more difficult to detect with a passive ferromagnetic detector.
There are ferromagnetic objects that are even more difficult to magnetize. These harder ferromagnetic objects, such as a bobby pin used in women's hair, may not be sufficiently magnetized even in the 1 to 5 Gauss field typically present near the entrance to the room which contains the MRI magnets. The magnetization curve for a bobby pin is shown in FIG. 3.
A possible solution may be had with an active magnetic detector device of the portal or wand configuration, meaning a detection device which contains permanent magnet or coils to generate a magnetic field. A field may be generated which is strong enough to magnetize even hard ferromagnetic objects. However the use of large magnetic fields is dangerous for personnel. For example, if a person has an intracranial aneurysm clip, a large applied magnetic field may twist the clip potentially resulting in a brain hemorrhage. Cardiac pacemakers may cease to generate timing signals or be reprogrammed in a large magnetic field. Because of these risks as well as many others, the Food and Drug Administration and the International Commission on Non-Ionizing Radiation Protection and others have defined 5 Gauss as the maximum magnetic field exposure safe for the general public. As previously noted, a 5 Gauss applied magnetic field is not sufficient to magnetize medium hard or hard magnetic material.